The invention relates to fuel cell systems and gas sensors employed with same. The invention also relates to metal oxide semiconductor gas sensors and a method for quickly rendering MOS gas sensors operational.
For safety purposes, gas-sensing instruments are used in many industrial applications such as in fuel cell systems whose feedstocks are flammable gases. It is well known that many gas sensorsxe2x80x94metal oxide semiconductor (MOS) based sensors in particularxe2x80x94suffer from environmental dependencies. That is, ambient temperature and relative humidity substantially affect their sensitivity. For example, one commercially available MOS sensor model is the Figaro TGS821 hydrogen sensor. Due to the combination of this sensor""s environmental dependencies and the environmental uncertainties to which it will be exposed to in certain fuel cell applications, a sensor reporting a reading of 526 PPM of hydrogen might actually be exposed to a true concentration ranging between 182 and 1627 PPM. In certain fuel cell applications, the lower reading would be regarded as being well below alarm-level whereas the higher reading would be regarded as being well above. This 8.9:1 range of uncertainty is the source of much frustration with uncompensated MOS gas sensors.
Accordingly, many designers of gas sensing instruments elect to compensate for MOS gas sensors"" environmental dependencies. The conventional wisdom is that this requires a microprocessor, firmware (software), and lookup charts. However, dependence upon firmware being perpetually executed without error in a microprocessor-based circuit greatly complicates efforts to design a highly reliable, fail-safe gas-sensing instrument. Furthermore, the conventional method produces compensation factors that are inexact approximations of the required values.
Attention is directed toward the following U.S. patents, which are incorporated herein by reference: U.S. Pat. No. 5,716,506 to Maclay et al.; U.S. Pat. No. 4,313,338 to Abe et al.; U.S. Pat. No. 4,801,211 to Yagi et al.; U.S. Pat. No. 6,126,311 to Schuh; and U.S. Pat. No. 5,969,231 to Qu et al.
U.S. Pat. No. 5,716,506 to Maclay et al. discloses (see Col. 1) a gas sensor that compensates for relative humidity and temperature of the air in the detection of a predetermined gas in a microfabricated electrochemical sensor.
U.S. Pat. No. 4,313,338 to Abe et al. relates to a gas sensing device comprising a resistive film formed of ultra fine particles of a metal oxide (Col. 4, lines 10-15). The gas sensing device includes (Col. 7, line 43-Col 8, line 65) a temperature sensing element for maintaining the temperature of the gas sensitive element constant. U.S. Pat. No. 4,313,338 also discloses obviating the problem of water vapor obstructing the successful measurement of the concentration of gas by using a single gas sensing element to sense both the concentration of water vapor and the concentration of isobutane gas (see Col. 8, line 47-Col. 9, line 11). The gas sensing element is heated up to 300 degrees Celsius during the measurement of the concentration of the isobutane gas and is cooled down to the room temperature of 25 degrees C. during the measurement of relative humidity.
U.S. Pat. No. 4,801,211 to Yagi et al. discloses (see Abstract) a humidity sensor that, when temperature corrected, indicates a dew point at a fixed temperature. By adjusting this fixed temperature dew point output according to a sensed temperature, the dew point can be detected. FIG. 2 shows all analog circuitry. The sensor is made of metal oxide ceramic material (see Col. 4, lines 44-46).
U.S. Pat. No. 6,126,311 to Schuh discloses (see FIG. 4) a sensor that outputs dew point, ambient temperature, and relative humidity. This patent discloses (see Col. 1, lines 14-20) that the relative humidity and dew point of a gaseous sample are closely related by well known algorithms for converting dew point and ambient temperature to relative humidity or converting relative humidity and ambient temperature to dew point. This patent also indicates (see Col. 2, lines 19-23) that a group of prior art sensors measure the relative humidity of an ambient environment as opposed to dew point, and that relative humidity and dew point are easily converted from one to the other with a measurement of the ambient air temperature.
U.S. Pat. No. 5,969,231 to Qu et al. discloses a sensor for monitoring the concentration of moisture and gaseous substances in the air. Semiconductive metal oxides are used (see Col. 1).
Notwithstanding the prior art teachings noted above, none of these references singularly or in any permissible combination teach a simple approach for compensating gas sensor measurements for both humidity and temperature at the same time. Further, none of these references offer a reasonable solution to the problem of slow start-up times for these same sensors. It would be advantageous therefore, to be able to perform such compensation utilizing analog circuitry, which would be highly reliable and substantially fail-safe, and to further provide a method whereby these sensors could be rendered fully operational in a relatively short period of time.
As noted above, gas sensors are used, in various industrial applications, such as in the fabrication of fuel cells. For example, gas sensors configured to sense hydrogen can be employed to detect hydrogen fuel leaks or hydrogen fuel flow in the fuel cells. In this regard, attention is directed to commonly assigned U.S. patent application Ser. No. 09/322,666 filed May 28, 1999, listing as inventors Fuglevand et al., and which is incorporated by reference herein. This application discloses the particulars of how gas sensors can be employed in one form of a fuel cell system.
Metal oxide semiconductor sensors typically require some time to xe2x80x9cwarm upxe2x80x9d, stabilize and thereby be rendered operational. Still further, many MOS sensors have cross-sensitivities to non-target gases. For example, if the target gas is hydrogen and the selected sensor has a cross-sensitivity to alcohol (as does the Figaro TGS813, for example), then even a perfumed person walking by the sensor can provoke a hydrogen alarm. In light of this, the sensor used in fuel cell applications, to sense the presence of fuel is preferably selected with cross-sensitivity in mind. For example, the Figaro TGS821, which is used as the sensor 400 in the illustrated embodiment, is less susceptible than other commercially available models, including the Figaro TGS813, but this same effect can still cause problems during operation and startup.
In addition to the foregoing, this cross-sensitivity causes problems at A startup because high-molecular-weight volatile organic compounds (VOCs) can deposit onto the sensor element while the fuel cell is off (and the sensor element is cold). Upon startup, the volatile organic compounds begin to decompose and evaporate. The sensor can then sense these byproducts and can output false positives upon startup. Though the sensor element referenced above may reach an operating temperature within about a minute, it can take several additional minutes to boil-off or evaporate stubborn volatile organic compounds from off of the sensor and thereby obtain a stable reading.
In the case of the commercially available Figaro model TGS813 sensor, for example, the manufacturer recommends accelerating this process by applying 6 V to the heater for 60 seconds. This raises the sensor temperature to about 600xc2x0 C. in order to facilitate the burn-off of stubborn volatile organic compounds that would otherwise only slowly burn-off at operating temperatures of 500xc2x0 C. or less. The manufacturer does not provide similar information on the TGS821. This model is used to sense the presence of hydrogen fuel gas in fuel cell systems, for example. The manufacturer does, however, advise that the burn-off time can be accelerated for another of their sensors, the TGS812, by applying 6.5 V for 15 seconds. After this time period, 5 V is applied and, after waiting for another half-minute for the reading to stabilize, the sensor can be expected to provide reasonably valid data.
The sensor element inside the model TGS821 normally operates at 500xc2x0 C. and, according to the manufacturer, should not experience temperatures in excess of 600xc2x0 C. An input of 6.0 V to the sensor 400 will produce a 600xc2x0 C. equilibrium temperature. In practice, it takes about one minute of heating for the sensor element to ramp up and stabilize at either of these 500 to 600xc2x0 C. equilibrium temperatures.
It would be advantageous if the sensors noted above could be rendered operational in short periods of time. This would permit readings to be taken more promptly. In a situation where the target gas being sensed by the sensor is a fuel or is poisonous, for example, it can be particularly important, for safety reasons, to be able to take accurate readings in the shortest time period possible.